In a stretch blow molding machine, Japanese patent publication no. 2000-43131 discloses that extending movement of a stretch rod for stretching a preform is done by converting rotary movement of an electric motor to linear movement with using combination of a ball screw and a ball nut.
In the rotary-linear movement converter comprising a ball screw and a ball nut, it is necessary to provide a circulating path for balls in order to make the relative thrust movement of the ball screw and the ball nut smooth. Therefore, the circulating path for balls is protrusively formed on the side of the body of the ball nut, and this is the reason why it is difficult to make the ball nut simple and compact.
The balls in the thread receive a compressive stress in every extensive movement of the stretch rod by being caught in the thread, so balls often reach fatigue fracture from the compressive stress when the extensive movement is repeated for a long time. Besides, the ball screw has a problem of requiring a lot of maintenance such as oiling to prevent the heat from friction.
Moreover, when a preform is formed into a bottle by air blow with lifting a bottom mold into a cavity and clamping the bottom portion of the preform with a stretch rod, if the bottom mold collides the tip of the stretch rod, the impact acts on the ball screw as a compressive force, and this becomes a cause of a trouble such as bending of stretch rods. To prevent such a trouble, it requires a locating control, which controls both of the extending movement of the stretch rod and the lifting movement of the bottom mold together, and makes the stretch rod and the bottom mold stop at a preset point.
As a means for clamping the bottom portion of a preform, Japanese patent publication no. 2000-43131 discloses an apparatus comprising a bottom mold having a movable center pin in the center of the bottom mold, and the lifting means for the center pin comprises a screw shaft and a nut member. In this case, the apparatus has a problem as mentioned above.
The present invention solves the problem mentioned above by adopting a converting means comprising a combination of a magnetic screw shaft and a magnetic nut member, which enable to convert rotary movement into linear movement smoothly without contact, and provides a novel stretch rod extending device which enables to improve the precision of the stretch rod and simplicity and the durability of the device.
Moreover, the present invention also provides a novel bottom mold lifting device which enables to improve the precision of the bottom mold and simplicity and the durability of the device by adopting a converting means comprising a magnetic screw and a magnetic nut member as a lifting means for the bottom mold of the blow mold. As disclosed in Japanese patent no. 2685723, the usage of the converting means comprising a combination of a magnetic screw shaft and a magnetic nut member is limited for conveyors. The present invention connects a combination of a magnetic screw shaft and a magnetic nut member to a stretch rod or a bottom mold with a specific construction, and provides a novel stretch rod extending device and bottom mold lifting device, which are not obvious to be imagined from the conventional usage as conveyors.